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K. M. Bumsted O'Brien, D. Hickey; Photoreceptor Density Increases and the Rod Free Zone Area Decreases During Foveal Excavation in the Developing Pigeon Retina. Invest. Ophthalmol. Vis. Sci. 2010;51(13):5934.
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The pigeon retina has a specialization for high acuity, the fovea centralis, where there is a shallow pit overlying a region where rods are missing from the photoreceptor mosaic and cones are packed at their highest density. Currently, there is little information available concerning the morphological development of the fovea in birds. The aim of this study was to analyze the changes in retinal morphology and cell density during the development of the pigeon fovea.
A developmental series of eyes from embryonic and post-hatch (P) pigeons (n=38) (Columba livia) were fixed in 4% paraformaldehyde. Flat mounted retinas were used for photoreceptor density counts and rod opsin immunocytochemistry. Frozen sections were stained with Cresyl violet for morphological analysis.
The incipient fovea was identified at embryonic day 12 and characterized by a single layer of cones and thickening of the ganglion cell layer (GCL). The pit began to form at post-hatch week (Pwk) 1, appearing as a shallow depression in the GCL, 2.3mm nasal to the optic disc. The position of the pit realtive to the optic disc remained constant while foveal excavation continued. Concomitant with pit excavation, foveal cone density increased from 180,841 cells/mm2 at P0 to 328,564 cells/mm2 in the adult. There was a significant inverse correlation between decreases in the area of the rod free zone and increases in cone density (Spearman r = -0.897, P<0.0001).
These data demonstrate the pigeon fovea forms post-hatch with a large increase in foveal photoreceptor density, and a decrease in the rod free area occurring from P0 to adult. This morphological progression is similar to that reported for primates and indicates that the pigeon is an appropriate model to investigate mechanisms that mediate devlopment of a fovea.
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